Consolidate AI agent instructions into AGENTS.md

- Create AGENTS.md with unified agent instructions
- Update symlinks for .cursorrules, .windsurfrules, and CLAUDE.md to point to AGENTS.md
- Replace .github/copilot-instructions.md with symlink to AGENTS.md
- Centralize all AI agent configurations in single file

Co-Authored-By: Claude <noreply@anthropic.com>